Rhodok Sergeants. End of story.
Spears may suck against horses (or the spearman AI, I guess-- piercing damage is good against the horses themselves), but the Rhodok Sergeants who spawn with banded armor and glaives are downright nasty, particularly against cavalry-- they have quick footwork, long melee range and high damage.
And they have good armor and hit points if they get charged.
If you want foot soldiers on the battlefield full of cavalry, get some of them and bait the enemy cavalry through a pack of them and enjoy the lines of green __________ was killed by Rhodok Sergeant text.
In fact, I'd say that that's the one thing any Rhodok unit is uniquely good at; they have no horses, Vaegirs are better archers than Sharpshooters, Huscarls are better infantry on offense or defense against other footsoldiers.
But the Sergeants (the good ones with glaives, anyway) can chop down enemy cavalry better than any other unit in the game.